Routing traffic through a virtual router-based network switch

ABSTRACT

Methods and systems are provided for routing traffic through a virtual router-based network switch. According to one embodiment, a method for routing packets in a router includes establishing a flow data structure, which identifies a packet flow through a virtual router in the router. When a packet is received, a comparison is performed between a subset of at least one packet header associated with the packet and a subset of the flow data structure. If the subset of the packet header matches the subset of the flow data structure, then the packet can be hardware accelerated to a network interface. Otherwise, the packet may be either dropped or forwarded to a general purpose processor for processing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 10/163,079 filed on Jun. 4, 2002, which is hereby incorporated by reference for all purposes.

This application is also related to the following US patents and US patent applications, all of which are incorporated herein by reference in their entireties for all purposes:

U.S. Pat. No. 7,161,904, entitled, “SYSTEM AND METHOD FOR HIERARCHICAL METERING IN A VIRTUAL ROUTER BASED NETWORK SWITCH;”

Application Ser. No. 10/163,261, entitled, “NETWORK PACKET STEERING;”

U.S. Pat. No. 7,116,665, entitled, “METHODS AND SYSTEMS FOR A DISTRIBUTED PROVIDER EDGE;”

Application Ser. No. 10/163,071, entitled, “SYSTEM AND METHOD FOR CONTROLLING ROUTING IN A VIRTUAL ROUTER SYSTEM;” and

Application Ser. No. 10/163,260, entitled “SERVICE PROCESSING SWITCH”

COPYRIGHT NOTICE

Contained herein is material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction of the patent disclosure by any person as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all rights to the copyright whatsoever. Copyright © 2002-2007, Fortinet, Inc.

BACKGROUND

1. Field

Embodiments of the present invention generally relate to high performance network switches, and more particularly to routing traffic through a virtual router-based network switch.

2. Description of the Related Art

The use of networks, including LANs, WANs and the Internet continues to grow at ever increasing rates. Each day, more and more systems are becoming interconnected. This has created a need for high performance network switches on the part of network service providers. Many of the switches comprise multiple modules with many data flows between the modules themselves and between the interfaces to external networks. In some cases, these modules, including the processors residing on the modules can be partitioned into virtual routers, such as software running on the processors that emulates the functioning of an individual physical router. As a result of the combination of hundreds of thousands of data flows for the virtual routers in these network switches, there is a need for efficiently processing packet flows and for controlling the resources consumed within the network switch.

SUMMARY

Methods and systems are described for routing traffic through a virtual router-based network switch. According to one embodiment, a flow data structure identifying packet flows associated with multiple virtual routers in a virtual router-based network device is established. An incoming packet is received having at least one packet header. A subset of the packet header is compared to a subset of the flow data structure. If the subset the packet header matches the subset of the flow data structure, then the incoming packet is hardware forwarded via a network interface of the virtual router-based network device without intervention by a processor of the virtual router-based network device, otherwise the incoming packet is forwarded to software on the processor for flow learning.

According to another embodiment, a method is provided for validating micro-flows. A hardware accelerated micro-flow is established by configuring forwarding state information of a flow cache entry associated with the hardware accelerated micro-flow. The hardware accelerated micro-flow includes an identifier and an invalidation tag. Upon receiving an incoming packet that is part of the hardware accelerated micro-flow, the invalidation tag is compared to a value in an invalid tag table located by the identifier. The hardware accelerated micro-flow is invalidated when the value does not match the invalidation tag.

According to another embodiment, a method is provided for capping packet flow. A rate metering structure is associated with each of multiple micro-flows of a virtual router-based network device. A corresponding rate statistic is maintained for each micro-flow in the rate metering structures. Upon detecting that the corresponding rate statistic is exceeded for a micro-flow with which an incoming packet is associated, dropping the incoming packet.

According to another embodiment, a method is provided for limiting resource consumption in a virtual router-based network device. A flow metering structure is associated with a virtual router. Upon detecting a packet flow is to be assigned to the virtual router, a flow counter in the flow metering structure is incremented. The flow counter is then compared to a predetermined limit value and if the flow counter does not exceed the predetermined limit value then establishing the packet flow, otherwise refusing to establish the packet flow.

Other features of embodiments of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1 is a block diagram of a virtual router hardware and operating environment in which various embodiments of the present invention may be practiced;

FIG. 2 is a diagram providing further details of a packet forwarding engine environment according to one embodiment of the present invention;

FIG. 3 is a flowchart illustrating a method for routing packets using hardware acceleration according to one embodiment of the present invention;

FIG. 4 is a flowchart illustrating a method for invalidating a hardware accelerated packet flow according to one embodiment of the present invention;

FIG. 5 is a flowchart illustrating a method for packet flow capping according to one embodiment of the present invention; and

FIG. 6 is a flowchart illustrating a method for limiting resource consumption for hardware accelerated packet flows according to one embodiment of the present invention

DETAILED DESCRIPTION

Methods and systems are described for routing traffic through a virtual router-based network switch. In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is show by way of illustration specific exemplary embodiments in which the invention may be practices. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the present invention.

Some portions of the detailed description which follows are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the ways used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared and/or otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers or the like. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system or similar computing device that manipulates and transforms data represented as physical (e.g., electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

Embodiments of the present invention may be provided as a computer program product, which may include a machine-readable medium having stored thereon instructions, which may be used to program a computer (or other electronic devices) to perform a process. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), and magneto-optical disks, ROMs, random access memories (RAMs), erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), magnetic or optical cards, flash memory, or other type of media/machine-readable medium suitable for storing electronic instructions. Moreover, embodiments of the present invention may also be downloaded as a computer program product, wherein the program may be transferred from a remote computer to a requesting computer by way of data signals embodied in a carrier wave or other propagation medium via a communication link (e.g., a modem or network connection).

Abbreviations

The following abbreviations may be used in the detailed description that follows. If an abbreviation is used that does not appear in the list, the meaning as used by one of skill in the art is intended.

API Application Programming Interface DB Database CBR Control Blade Redundancy CEP Connection End Point CM Configuration Manager (CLI or SNMP) DML Distributed Messaging Layer IOCTL Input Output Control IPNOS IP Network Operating System IPSX IP Service eXchange LQ Logical Queue OM Object Manager OMCD Object Manager Configuration Database OMORI Object Manager Object Routing and Interface OMORIG Object Manager Object Routing and Interface Global OS Operating System RLS Resource Location Service RM Resource Manager VI Virtual Interface VPN Virtual Private Network VR Virtual Router Operating Environment

FIG. 1 illustrates a router operating environment 100 used in some embodiments of the present invention. Environment 100 includes an external network interface 102, a midplane interface 108 and at least one processing element 106 all communicably coupled through an internal network interface 104. In some embodiments, midplane interface 108 connects environment 100 to a midplane capable of interconnecting a plurality of environments 100 in a service module or a cabinet.

In one embodiment of the present invention, external network interface 102 is referred to as a line interface, and provides a media access interface to wired or wireless network. Examples of such line interfaces include Gigabit Ethernet, OC-12/STM-4 POS, OC-3/STM-1 POS and DS3C/DS3U/E3U interfaces. The invention is not limited to any particular type of line interface or network type.

In some embodiments, internal network interface 104 is a switch fabric interface. In one embodiment, the switch fabric interface is a 51.2 Gbps, 8-port, fully meshed, non-blocking switch fabric, with each port supporting a 6.4 Gpbs transfer rate. However, the invention is not limited to any particular type of switch interface or internal network interface 104.

In some embodiments, processing engines 106 provide specialized application processing within environment 100. In some embodiments, processing engine 106 can be a Virtual Routing Engine (VRE) capable of providing virtual router applications 116. In alternative embodiments, processing engine 106 can be a Virtual Service Engine (VSE) capable of providing services, such as firewall services and antivirus services. In further alternative embodiments, processing engine 106 can be an Advances Security Engine capable of providing data encryption services.

Processing engine 106, in some embodiments, includes a Packet Forwarding Engine (PFE) 110, processor 112 and memory 114. Processor 112 executes computerized instructions that form the various types of applications that can be run on a processing engine 106. In one embodiment of the present invention, processor 112 is a PowerPC 750CX from IBM Corp. In an alternative embodiment, processor 112 is a Hi/fn 7851. In a further alternative embodiment, processor 112 is a Hi/fn 6500. The invention is not limited to any particular type of processor. Additionally, in some embodiments of the present invention, processing engine 106 includes more than one processor 112. The invention is not limited to any particular number of processors 112.

PFE 110, in some embodiments, comprises circuits and logic that perform hardware assisted packet routing for a processing engine 106. In general, PFE 110 analyzes packets that arrive from the internal network interface or from a DMA interface with processor 112. PFE 110 then determines whether the packet can be hardware forwarded without the intervention of processor 112 or whether such intervention is required. Further details on the structure of PFE 110 and methods implemented within PFE 110 will be provided below.

Memory 114 is a memory capable of storing data and instructions for processor 112 and PFE 110. In some embodiments, processor 112 and PFE 110 share memory 114. In alternative embodiments, each of processor 112 and PFE 110 has dedicated memory.

FIG. 2 illustrates further details on a PFE 110 according to various embodiments of the present invention. In some embodiments, PFE 110 is partitioned into an ingress unit 220 and egress module 210. In some embodiments, the PFE ingress unit 220 includes a switch fabric interface ingress 224 that processes incoming packets from the internal network interface 104 and transfers them to the DMA Engine ingress 222. The PFE egress unit 210 processes outgoing packets from the DMA Engine egress 202 and transfers them to the internal network 104 using a switch fabric egress module 204. In some embodiments, both the ingress and egress units have direct access to the PE memory system 114 (FIG. 1). Further in some embodiments, the PFE 110 operates synchronously to the processor 112 interface and memory system 114 at 100 MHz.

In some embodiments, both the PFE ingress and egress units comprise an array of 32-bit packet processors 206 that share an on-chip write-back cache 212. In some embodiments, each ingress unit 220 and egress unit 210 include six packet processors 206. However, the invention is not limited to any particular number of packet processors. In one embodiment, the packet processor implements a RISC-like integer instruction set with special instructions for bit field extraction and merging and for unaligned block data transfers. According to one embodiment, each packet processor 206 operates on a different packet and hardware interlocks maintain packet order. In some embodiments, the ingress processors share common micro-code for ingress processing and the egress processors share common micro-code for egress processing. In some embodiments, the PFE 110 memory maps the ingress and egress instruction stores and supports micro-code updates through Programmed Input/Output (PIO) write transactions.

In operation, the PFE ingress unit 220 passes forwarding state to the DMA Engine 202 that incorporates this state into a packet receive descriptor. This forwarding state indicates whether the processor 112 should software forward the packet or the packet may bypass the processor and the PFE 110 can hardware forward the packet. In some embodiments, the forwarding state also includes a 20-bit index into a forwarding transform cache that describes PFE processing per packet flow. For software forwarding packets, a receive descriptor is pushed onto the DMA ingress descriptor queue. For hardware forwarded packets, the descriptor will bypass the DMA ingress queue and be pushed directly onto the DMA egress descriptor queue as a transmit descriptor.

FIGS. 3-6 are flowcharts illustrating methods for providing hardware accelerated packet routing and for controlling resources related to such hardware accelerated packet routing. The methods to be performed by the operating environment constitute computer programs made up of computer-executable instructions. Describing the methods by reference to a flowchart enables one skilled in the art to develop such programs including such instructions to carry out the methods on suitable computers (the processor of the computer executing the instructions from computer-readable media). The methods illustrated in FIGS. 3-6 are inclusive of the acts required to be taken by an operating environment executing an exemplary embodiment of the present invention.

FIG. 3 illustrates a method for performing packet forwarding using hardware packet acceleration. The method is invoked when a packet is received by a packet processor, such as packet processor 206. In some embodiments, the method begins by performing ingress rate limiting (block 302). Further details on rate limiting will be provided below. Next, ingress statistics are updated (block 304).

Next, in some embodiments of the present invention, the packet is classified (block 306). Packet classification involves looking at one or more of the various protocol layer headers. As those skilled in the art will appreciate, packets are typically encapsulated with various protocol headers that define sources, destinations and applications that are to process the data in the packet. For example, layer 2 (L2) is typically a data link layer that specifies the organization of data into frames and how to send frames over a network; layer 3 (L3) is a network layer that specifies addressing assignments and how packets are forwarded and layer 4 (L4) is a transport layer that specifies how to reliably handle data transfer. For example TCP/IP occupies L3 and L4 in this reference model. In some embodiments, layer 1 (L1, also referred to as LQ) includes a header that specifies an LQ ID and LQ protocol to use to route packets internally within a router. The LQ ID is used to identify a particular VR that is to receive the packet.

In some embodiments, the PFE ingress unit supports two basic forms of packet classification. One is flow-based, using various fields of the LQ header along with fields in the L3/L4 headers to identify a particular VR flow (also referred to as a micro-flow). The other form uses the upper bits of the IP address or MPLS label to index a table of flow indices. According to one embodiment, the host software controls which classification form the PFE uses by programming different micro-code into the ingress instruction store. In one embodiment, in both forms of packet classification, the classification result is a 20-bit forwarding index that the hardware uses to select the correct packet transformation.

Each flow ID cache entry stores the LQ ID, LQ protocol, L3, and L4 fields that identify a particular VR flow along with state indicating whether to hardware or software forwarding packets belonging to the micro-flow. The PFE ingress unit generates an index into the flow ID cache by hashing the incoming packet's LQ ID, LQ protocol, L3, and L4 header fields. It then looks-up the indexed cache entry and compares the packet micro-flow ID fields to the cached micro-flow ID fields. On a cache hit, the FwdAction field of the cache entry indicates whether to software or hardware forward the packet. On a cache miss, the ingress controller allocates a cache entry and forwards the packet to software on processor 112 for flow learning.

TABLE 1 Flow ID Cache Block Word Bits Name Description 0 31 Valid Indicates the flow ID block ID is valid.  30:29 FwdAction Indicates forwarding action: 0: Pending 1: Software forward 2: Hardware forward 3: Hardware forward SPF 28 Reserved 27 AgeEnable Enable for Flow Aging 26 Reserved  25:16 VRGroupID The VR invalidation group ID. The hardware uses this ID to select an entry in the Invalidation Tag table to determine if this FCB should be invalidated 15:0 FlowAgeTime Software uses this field to set the flow lifetime in seconds. Hardware translates the flow lifetime into an expiration timestamp after which the flow will become invalid 1  31:16 PendingTag The tag uniquely identifies a flow from other flows which have resided in the same FCB. The tag is incremented each time the FCB is replaced by a new flow. One new flow and HW packets the SrcChan in the SF header is replaced with the tag. The tag in the FCB is compared with the tag in a FCB_UPDATE message. The tags must match for the PFE to accept the update. 15:0 VRInvTag This field holds the VR group invalidation tag that was current at the time the FCB update message was received. 2  31:24 FlowCapTag Used to determine if this flow has been counted for the current interval in flow cap processing.  23:16 LQ_PROTO LQ protocol ID field 15:0 LQ_ID Destination LQID field 3 31:0 L3 L3 header fields required for flow 4 31:0 classification 5 31:0 MPLS {Label, Exp} IP {Protocol, TOS, Src Addr, Dst Addr} 6 31:0 L4 L4 header fields required for flow classification TCP/UDP {src Port, Dst Port} IPSec {SPI} 7 31:0 L2 PPP {Protocol}, L2TP {Tunnel ID, Session ID}

TABLE 2 Flow ID L3 Formats Word Bits Name Description MPLS 3  31:24 Reserved 23:4 Label MPLS Label field  3:1 Exp MPLS Exp field 0 Stack MPLS Stack field 4 31:0 Reserved 5 31:0 Reserved IPv4 3  31:16 Reserved 215:8  Proto IP header protocol field  7:0 TOS IP header TOS field 4 31:0 Src IP header source address field 5 31:0 Dst IP header destination address field

TABLE 3 Flow ID L4 Formats Word Bits Name Description TCP/UDP 6  31:16 Src TCP/UDP header source port field 15:0 Dst TCP/UDP header destination port field IPSec 6 31:0 SPI IPSec AH or EPS header SPI field

TABLE 4 L2 Tunnel/Detunnel formats Word Bits Name Description PPP 7 31:8 Reserved  7:0 Protocol PPP header protocol field L2TP (LNS) 3  31:16 Reserved 15:8 Proto Encapsulated IP header protocol field  7:0 TOS Encapsulated IP header TOS field 4 31:0 Src Encapsulated IP header source address field 5 31:0 Dst Encapsulated IP header destination address field 6  31:16 Src Encapsulated TCP/UDP source port. Reserved if IP proto != TCP or UDP 15:0 Dst Encapsulated TCP/UDP destination port. Reserved if IP proto != TCP or UDP 7  31:16 Tunnel ID L2TP header Tunnel identification field. 15:0 Session ID L2TP header Session identification field L2TP (LAC) 7  31:16 Tunnel ID L2TP header Tunnel identification field. 15:0 Session ID L2TP header Session identification field

According to one embodiment, when the PFE controller detects a new flow, in attempts to allocate one of four FCBs selected using its hashed flow index. If the PFE finds an available FCB, it replaces the FCB tag fields, sets the Fwdction field to pending and increments the FCB pending tag. The PFE then forwards the packet to software using a descriptor marking of FWD_NEW. At some later time, the host software sends a control packet containing an FCB_UPDATE message for this flow, which sets up the FCB.

According to one embodiment, if the PFE is unable to allocate an FCB, it forwards the packet to software using a descriptor marking of FWD_COLLISION. This indicates to software that the packet's flow collided with another currently active flow in the FCB cache.

In one embodiment, during the time between the first packet's arrival and the arrival of the FCB_UPDATE message for that flow, the PFE forwards all packets of that flow to software marked with a FWD_NEW descriptor. In the case that another new flow replaces FCB before an FCB_UPDATE message arrives, the PFE uses the FCB's Pending Tag field to uniquely identify the flow for the FCB_UPDATE messages.

According to one embodiment, for each new flow, the PFE ingress controller replaces the FCB flow tag fields and increments the pending tag. Upon the arrival of an FCB_UPDATE message the PFE compares the pending tag in the FCB and the message, and if they are equal accepts the message. If the pending tags differ when the FCB_UPDATE message arrives, the PFE ignores the update message. In this way, the PFE ignores stale FCB update message.

If the packet can be hardware forwarded, the packet processor then performs transform block processing (block 310). The PFE maintains a table of Transform Control Blocks (TCBs), which direct how the egress controller processes outgoing-packets. The egress controller uses the 20-bit forwarding index, carried by the DMA descriptor, to select a transform control block from the table before processing packets. In one embodiment, each transform control block entry contains 64-bytes formatted as described in the table below.

TABLE 5 Transform Control Block Word Bits Name Description 1 31 NAT_IP Perform NAT on IP addresses. 30 DropCpuPkt If this bit is set and the Pkt desc is HW_COH the packet is dropped 29 NAT_TCP Perform NAT on TCP/UDP port addresses 28 ReplaceRM Replace Rate-Marking field in SF header. 27 ReplaceID Replace IP header ID field with incremented PktID. 26 ValidCRC Validate IP header checksum. 25 DecrTTL Decrement the IP or MPLS header TTL value 24 ReplacePRI Replace Priority field in SF header.  23:16 TOS/EXP IP TOS/MPLS EXP replacement value 15:8 TOS/EXP Enables for IP TOS/MPLS EXP Enables replacement. (Set high for replace bit)  7:4 MPLS MPLS Operation Code Operation 0: NOP 1: PUSH 2: POP_PEEK 3: POP_FWD 4: SWAP 5: POP_L2VPN_NULL 6: POP_L2VPN_CTRL  3 PWE3 Enable PWE3 special case handling of L2 packets  2 PWE3 Control PWE3 control word should be added. Used when CW is “optional.”  1:0 Reserved 2 31:0 StatsOutPtr0 Memory pointer to egress statistics block 0. 3 31:0 StatsOutPtr1 Memory pointer to egress statistics block 1 (always assumed enabled). 4  31:16 HdrOffset Indicates the number of bytes before the start of payload when an application specific header is located. Used for PPPoE. Also used for detunneling, indicates the number of bytes to strip before detunneling. 15:0 HdrLen Byte length of the transform header. 4 31:0 Src Encapsulated IP header source address field 5 31:0 HdrPtr Memory pointer to the transform header data 6 31:0 NAT.IPSrc IP source address NAT replacement value 7 31:0 NAT.IPDst IP destination address NAT replacement value 8  31:16 NAT.TCPSrc TCP/UDP source port NAT replacement value 15:0 NAT.TCPDst TCP/UDP destination port NAT replacement value 9 31:0 PktIdPtr Memory pointer to packet ID value 10 31:0 MeterOutPtr0 Memory pointer to egress metering control block 0. 11 31:0 MeterOutPtr1 Memory pointer to egress metering control block 1. 12 31:8 Reserved  7:0 EgressQosIndex Mode and memory pointer to the egress QoS translation table 13 31:0 L3HeaderPtr Memory pointer to the L3 encapsulation header. 14 31:0 L3HeaderSize Size of the L3 encapsulation header. 15  31:16 FCBTag The value of the corresponding FCB pending tag must be written here to associate the TCB with the flow. A value of 0 needs to be written in prefix mode. 15:0 TCPChkAdj TCP Checksum adjustment for TCP transforms.

According to one embodiment, to update a Transform Control Block (TCB), host software sends a control packet containing a PFE_EGRESS_WR message with an address parameter that points to the new TCB. Software should issue the TCB update control packet before issuing the packet being forwarded. This ensures that the forward packet is processed according to the updated TCB.

In some embodiments, several fields may be used to maintain packet order and associate the TCB with a specific flow. In flow mode, where several NEW packets for a flow could be sent to the CPU, there is a danger that once the CPU updates the TCB and FCB a packet could be hardware forwarded while the CPU still has packets for that flow. In one embodiment, packet order is enforced by the TCB. When the TCB is written the DropCpuPkt bit should be zero, this will allow the CPU to send the NEW packets it has for that flow. However, when the first FWD_HW packet is seen with this bit clear, the packet forwarding engine (e.g., packet forwarding engine 110) will update the TCB and set this bit. Subsequent packets from the CPU (recognized because they are marked FWD_HW_COH) will be dropped. In alternative embodiments, packet order may be maintained by a conflict cache in the DMA engine.

There is also a consistency check performed between the FCB and the TCB. On ingress the SF header, SrcChan is replaced with the PendingTag field of the FCB, on egress, the SrcChan is compared against the FCBTag field of the TCB. If the tags mismatch, the packet is dropped. For prefix mode, the SrcChan is replaced with zero and the FCBTag field is initialized to zero.

Next, a packet processor executing the method performs header transformation (block 312). In its simplest form, the packet header transformation involves the replacement of some number of header bytes of an ingress packet with some number of bytes of replacement header data. Under the control of a TCB, the PFE egress unit can selectively replace and recompute specific fields in a small set of protocol headers.

The PFE egress unit begins the header transform by stripping the incoming packet's SF header along with the number of bytes indicated by the SF header offset field. At that point, the controller will begin copying bytes from the buffer pointed to by the TCB's HdrPtr field into the egress packet buffer. The PFE will copy the number of new header bytes defined by the TCB's Hdrlen field.

After performing this header replacement, the PFE then goes through the TCB enable bits to determine what other header transformations need to be made.

Next, egress rate limiting is performed (blocks 314, 316). Further details on rate limiting are presented below.

Finally, egress statistics are updated (blocks 318, 320) and the method returns to block 302 to await reception of another packet.

FIG. 4 is a flow chart illustrating a method for invalidating a hardware accelerated packet flow according to one embodiment of the present invention. The method begins by establishing a packet flow for a virtual router (block 402). The establishment of a packet flow has been described above with reference to FIG. 3.

Next, the method receives a packet associated with the flow (block 404). Upon receiving a packet, the method determines if the flow should be invalidated or not (block 406). In some embodiments of the present invention, a flow cache block (FCB) describing the flow maintains a VR Group ID value. This value is compared to a tag entry in a VR group invalidation table. If the values are different, the flow is invalid and is marked as such (block 408). The packet may be dropped or it may be forwarded to software running on processor 112 for reestablishment of a flow or other processing. Otherwise, the flow is valid and hardware acceleration continues for the packet (block 410).

In some embodiments, the VR group invalidation is typically incremented (thereby causing the FCB value and the VR group invalidation table value to differ) upon the occurrence of one of several events. One event is flow expiration. In some embodiments a PFE ingress controller applies a flow expiration timestamp to each new flow at the time software establishes the flow. Software defines the flow lifetime by setting the FCB FlowAgeTime field with the number of seconds the flow should remain active. The FCB AgeEnable field is also set for flow aging to be applied.

When a packet arrives for a valid flow, the PFE ingress controller compares the incoming packet's assigned timestamp with the FCB expiration timestamp. If the packet's timestamp exceeds the flow's expiration timestamp, then the PFE will invalidate the flow and send the packet to software with FwdCtl-FWD_New.

In addition to flow aging, the PFE microcode may support a software-initiated flow invalidation scheme. According to one embodiment, at the time software establishes a new flow with an FCB_UPDATE, it assigns the flow to one of 1024 VR invalidation groups by setting the FCB VRGroupID field. The PFE maintains an invalidation tag for each of the VR groups in a table, during the FCB_UPDATE the PFE copies the associated invalidation tag from the table and stores it into the new flow's FCB. Each time a packet arrives for the flow, the PFE ingress controller compares the FCB invalidation tag with the tag value in the associated invalidation table entry. If the PFE detects that an invalidation event has occurred for that flow, then the flow is re-established as a new flow. Software can invalidate all member flows of a VR group by changing the value of the invalidation tag in memory with a MEM_WR command to the appropriate address.

In some embodiments, VR Group index 0 is reserved for pending flows and the tag value in the invalidation table must be zero.

TABLE 6 VR Group Invalidation Tag Entry Format Word Bits Name Description 0  31:16 Reserved 15:0 VRInvTag VR group invalidation tag. 1 31:0 Reserved

Additionally, in some embodiments, when software sets the FCB's FlowAction field to hardware forward SPF, the PFE performs special TCP header checks before hardware forwarding packets in this flow. If the PFE detects SYN, FIN or RST flags set, then it pushes the packet to software for SPF state machine processing. Also, a FIN or RST flag will automatically invalidate the FCB for subsequent packets.

FIG. 5 is a flowchart illustrating a method for packet flow capping according to one embodiment of the present invention. The method begins by associating a meter structure to a packet flow (block 502). The meter structure may contain various statistics associated with the flow, including packet counts for a time interval. Next, a system executing the method receives a packet (block 504). In response, a packet counter in the meter structure is incremented.

Next, the packet count is compared to a predetermined maximum value to determine if the flow has exceeded the allowable packet count (block 506). If the maximum count is exceeded, the packet can be dropped (block 508). Otherwise, the packet can be processed as part of the normal hardware accelerated flow (block 510).

In some embodiments, the PFE egress unit independently rate limits ingress and egress packets, if enabled. As part of rate limiting, the PFE meters, marks and drops packets. The PFE performs ingress rate limiting before header transformation and performs egress rate limiting after header transformation. Software controls metering and rate marking using a combination of Metering Control Blocks (MCBs) and fields in the TCB and ingress statistics blocks.

In some embodiments, the PFE implements both ingress and egress rate metering and marking according to the two-rate three color marker (trTCM) definition in RFC 2698. Per this definition, in color-blind mode, the PFE marks the drop precedence color of a packet a Green if it does not exceed the CBS, Yellow if it exceeds the CBS but not the PBS, and Red if it exceeds both the CBS and PBS. The packet's color is encoded into the rm field of the LQ header. The PFE increments the C and P buckets by the CIR and PIR values, respectively, in 1 ms intervals.

The PFE egress unit may optionally drop Yellow or Red packets or may color packets for a downstream dropper. The RateInCtl and RateOutCtl fields of the TCB control whether and how to drop packets on ingress and egress rate limiting.

A set of Metering Control Blocks (MCBs) maintained in system memory contain per flow (VR, VI or ACL) trTCM parameters. Table 7 defines the MCB data structure according to one embodiment of the present invention. Hardware provides three logical metering units: VI-based ingress metering, flow-based ingress metering and flow-based egress metering. The TCB contains two MCB pointers for flow-based metering. The VI-based MCB pointer is contained in the VI-based stats block, which is discussed in further detail in the attached Appendix.

TABLE 7 Metering Control Block Word Bits Name Description 0 31:0 Green_bytes Bottom 32 bits of green-metered (lower) bytes count 1 31:0 Ctokens Number of bytes in C token bucket 2 31:0 Ptokens Number of bytes in P token bucket 3 31:0 Metered_pkts Bottom 32 bits of metered packet (lower) count. 4 31:0 Yellow_bytes Bottom 32 bits of yellow-metered (lower) bytes count. 5 31:0 Red_bytes Bottom 32 bits of red-metered (lower) bytes count. 6 31:0 Timeslot 1 ms timeslot value 7 31:0 Reserved 8 31:0 CIR Committed information rate in bytes/timeslot 9 31:0 PIR Peak information rate in bytes/timeslot 10 31:0 CBS Committed burst size in bytes 11 31:0 PBS Peak burst size in bytes 12  63:32 Metered_pkts Upper32 bits of metered packet (upper) count 13  63:32 Green_bytes Upper 32 bits of green-metered (upper) bytes count 14  63:32 Yellow_bytes Upper 32 bits of yellow-metered (upper) bytes count. 15  63:32 Red_bytes Upper 32 bits of red-metered bytes (upper) count.

According to one embodiment, software controls where and how the hardware accesses MCBs by setting up arrangements of MCB pointers. The MCB pointer data structure contains a 32-byte aligned memory pointer along with mode control bits as detailed in the table below. In its simplest form, the pointer field indicates the memory location of a single MCDB. In its most complex mode, the pointer indicates the location of an ordered array of up to 8 MCB pointers. When the hardware loads an MCB pointer array, it performs metering and rate marking starting with the first MCB pointer and continuing as directed by the NextPointer field in the MCB pointer. In one embodiment, software can disable rate marking completely by setting all 4 bytes of the MCB pointer to 0.

TABLE 8 MCB Pointer Format Bit Field Name Description 31:5  Memory Pointer This field contains a memory pointer to an MCB, an MCB pointer array or a Rate Marking Translation Table. The Metering Mode field determines which mode to use. 4:3 Metering Mode This field determines to what structure the Memory Pointer field points: 0: MCB - Color Blind 1: MCB - Color Aware 2: MCB Array 3: Reserved 2:1 Drop Policy This field indicates the traffic policing policy: 0: No dropping 1: Drop on red marking only 2: Drop on yellow or red marking 3: Reserved 0 Next Pointer This field indicates whether the hardware should continue to the next MCB pointer in an array: 0: Stop after the current pointer 1: Continue to the next MCB pointer in the array.

It should be noted that depending upon the particular needs, in accordance with various embodiments, metering can be applied at a packet and/or byte level based on the number of packets or bytes transferred in the flow.

FIG. 6 is a flow chart illustrating a method for limiting resource consumption for hardware accelerated packet flows according to one embodiment of the present invention. The method begins by associating a flow meter structure to a virtual router (block 602). Next, the system executing the method determines that a new flow is to be established for the virtual router (block 604). The system then checks to see if the flow count (i.e., the number of flows associated with the VR) would exceed a predetermined value (block 608). If so, the new packet flow is invalidated (block 610). Alternatively, the packet flow need not be created. Otherwise, hardware accelerated packet flow is established.

The flow cap feature is intended to allow the system to restrict the number of flows in the flow cache a particular VR can consume. Packets are associated with a flow cap structure based on LQID, an index into the flow cap table is located in the ingress statistics block.

The FlowCap field of the flow cap structure is used to set the maximum number of flows allowed and to disable flow cap processing with a value of 0. If flow cap processing is disabled no further processing is performed. Otherwise, the flow cap structure is checked to see if the current time interval has expired, if the structure needs to be initialized or if there is a mismatch between the FlowCapTags in the structure and the FCB. When software sets up a flow cap structure, the ExpTime field is initially set to 0. This indicates to the microcode that the structure needs to be initialized and timer expiration processing will be performed. When the timer has expired, the flow cap structure will be updated for the next interval, FlowCount will be copied to PrevMax, FlowCount will be set to 1 to count the current flow, FlowCap Tag will be incremented and ExpTime will be set to current_time+Expinterval. When a flow is counted due to expiration or tag mismatch the FCB will be updated to copy the FlowCapTag from the flow cap structure to the FCB. If a flow with a tag mismatch is processed, but the flow cap has been reached then the FCB will be invalidated and the packet discarded.

In one embodiment, new flows are also counted; and if the flow cap is exceeded the flow will not be established.

To insure that timer expiration is correctly identified, in one embodiment, the ExpInternal is set to less than half the full range (2G). With time units of 1 ms, this allows for up to 24 days for the maximum interval, far greater than expected for actual use.

TABLE 9 Flow Cap Structure Word Bits Name Description 0 31:0 FlowCount Active flow count for the current time interval 1 31:0 PrevMax Active flow count from the previous time interval 2 31:0 ExpTime Time stamp when this interval will be over 3  31:24 FlowCapTag Tag to match against FCB to determine if flow has been counted 23:0 Reserved 4 31:0 ExpInterval Length of the flow count interval in milliseconds 5 31:0 FlowCap Maximum number of flows allowed (0 disables flow cap) 6 31:0 Reserved 7 31:0 Reserved

In some embodiments of the present invention, a multiprocessor system that uses a shared memory, a cache lock bit is used to enforce ownership of a shared data structure, such as one of the data structures described above or in the attached Appendix. When a first processor, such as packet processor 206 loads a data structure, the lock bit is set in the cache tag store. A second processor requesting the data structure is denied access until the lock bit is reset. The lock bit is reset when the first processor completes an update of the data structure. When requesting access to the data structure, the first processor performs a “load-memory-lock,” and when complete, the first processor performs a “store-memory-unlock.” Accordingly, only one processor may update a data structure in cache at a time. Data structures include metering control blocks and status blocks. The cache lock bit is included within the cache tag store for each data element in the cache.

CONCLUSION

Systems and methods for hardware accelerated packet routing are disclosed. Although specific embodiments have been illustrated and described herein, the foregoing description of specific embodiments reveals the general nature of the invention sufficiently that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the generic concept. Therefore such adaptations and modifications are within the meaning and range of equivalents of the disclosed embodiments. The phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention embraces all such alternatives, modifications, equivalents and variations as fall within the spirit and scope of the appended claims. 

1. A method comprising: maintaining a flow cache having a plurality of flow ID cache block entries each identifying one of a plurality of current virtual router (VR) flows through a VR-based network device and corresponding forwarding state information; receiving an incoming packet at a processing engine of a plurality of processing engines of the VR-based network device; an ingress unit of a packet forwarding engine (PFE) associated with the processing engine determining whether the incoming packet is associated with a VR flow of the plurality of current VR flows by attempting to retrieve a flow ID cache block entry of the flow cache identified by an index based on one or more of (i) an Internet Protocol (IP) address, (ii) a Multiprotocol Label Switching (MPLS) label, and (iii) a destination logical queue (LQ) ID field, a LQ protocol ID field, one or more layer 3 (L3) header fields and one or more layer 4 (L4) header fields associated with the incoming packet; if it is determined that the incoming packet is associated with the VR flow, determining, based on the corresponding forwarding state information of the retrieved flow ID cache block entry, whether the incoming packet can be hardware forwarded or whether the incoming packet is to be software forwarded; if it is determined that the incoming packet can be hardware forwarded, then (i) determining one or more packet transformations that are to be applied to the incoming packet by an egress unit of the PFE as a result of the incoming packet's association with the VR flow, (ii) the egress unit applying the one or more packet transformations to the incoming packet, and (iii) hardware forwarding the incoming packet without intervention by a processor of the VR-based network device via a network interface of the VR-based network device; otherwise, if it is determined that the incoming packet cannot be hardware forwarded, then software forwarding the incoming packet via the processor; if it is determined that the incoming packet is not associated with any of the plurality of current VR flows, (i) identifying the existence of a new VR flow, (ii) allocating a new flow ID cache block entry within the flow cache for the new VR flow and (iii) forwarding the incoming packet to software on the processor for flow learning.
 2. The method of claim 1, further comprising: establishing a hardware accelerated flow within the flow cache, the hardware accelerated flow having associated therewith an identifier and an invalidation tag; if the packet is determined to be part of the hardware accelerated flow, then retrieving a value from an invalid tag table by indexing into the invalid tag table with the identifier and comparing the invalidation tag to the retrieved value; and invalidating the hardware accelerated flow when the retrieved value does not match the invalidation tag.
 3. The method of claim 1, further comprising: associating a rate metering structure with a VR flow of the plurality of current VR flows; maintaining a rate statistic within the rate metering structure indicative of a number of packets or bytes associated with the VR flow that have been observed during a predefined time interval; and enforcing a maximum packet or byte rate cap for the VR flow by dropping the packet if the packet would cause the rate statistic to exceed the maximum packet or byte rate cap for the predefined time interval.
 4. The method of claim 3, further comprising: associating a flow metering structure with a VR of a plurality of VRs of the VR-based network device; for each VR flow assigned to the VR, incrementing a corresponding flow counter in the flow metering structure; comparing the corresponding flow counter to a predetermined limit value; if the corresponding flow counter does not exceed the predetermined limit value then establishing the new VR flow, otherwise refusing to establish the new VR flow.
 5. The method of claim 1, wherein the processing engine comprises a Virtual Service Engine (VSE) configured to provide one or more services.
 6. The method of claim 5, wherein the VSE provides antivirus services.
 7. The method of claim 5, wherein the VSE provides firewall services.
 8. The method of claim 1, wherein the processing engine comprises an Advanced Security Engine (ASE) capable of providing data encryption services.
 9. A method comprising: a step for maintaining a flow cache having a plurality of flow ID cache block entries each identifying one of a plurality of current virtual router (VR) flows through a VR-based network device and corresponding forwarding state information; a step for receiving an incoming packet at a processing engine of a plurality of processing engines of the VR-based network device; a step for, an ingress unit of a packet forwarding engine (PFE) associated with the processing engine, determining whether the incoming packet is associated with a VR flow of the plurality of current VR flows by attempting to retrieve a flow ID cache block entry of the flow cache identified by an index based on one or more of (i) an Internet Protocol (IP) address, (ii) a Multiprotocol Label Switching (MPLS) label, and (iii) a destination logical queue (LQ) ID field, a LQ protocol ID field, one or more layer 3 (L3) header fields and one or more layer 4 (L4) header fields associated with the incoming packet; a step for, if it is determined that the incoming packet is associated with the VR flow, determining, based on the corresponding forwarding state information of the retrieved flow ID cache block entry, whether the incoming packet can be hardware forwarded or whether the incoming packet is to be software forwarded; a step for, if it is determined that the incoming packet can be hardware forwarded, (i) determining one or more packet transformations that are to be applied to the incoming packet by an egress unit of the PFE as a result of the incoming packet's association with the VR flow, (ii) the egress unit applying the one or more packet transformations to the incoming packet, and (iii) hardware forwarding the incoming packet without intervention by a processor of the VR-based network device via a network interface of the VR-based network device; otherwise, a step for, if it is determined that the incoming packet cannot be hardware forwarded, software forwarding the incoming packet via the processor; a step for, if it is determined that the incoming packet is not associated with any of the plurality of current VR flows, (i) identifying the existence of a new VR flow, (ii) allocating a new flow ID cache block entry within the flow cache for the new VR flow and (iii) forwarding the incoming packet to software on the processor for flow learning.
 10. The method of claim 9, further comprising: a step for establishing a hardware accelerated flow within the flow cache, the hardware accelerated flow having associated therewith an identifier and an invalidation tag; a step for, if the packet is determined to be part of the hardware accelerated flow, retrieving a value from an invalid tag table by indexing into the invalid tag table with the identifier and comparing the invalidation tag to the retrieved value; and a step for invalidating the hardware accelerated flow when the retrieved value does not match the invalidation tag.
 11. The method of claim 9, further comprising: a step for associating a rate metering structure with a VR flow of the plurality of current VR flows; a step for maintaining a rate statistic within the rate metering structure indicative of a number of packets or bytes associated with the VR flow that have been observed during a predefined time interval; and a step for enforcing a maximum packet or byte rate cap for the VR flow by dropping the packet if the packet would cause the rate statistic to exceed the maximum packet or byte rate cap for the predefined time interval.
 12. The method of claim 11, further comprising: a step for associating a flow metering structure with a VR of a plurality of VRs of the VR-based network device; a step for, for each VR flow assigned to the VR, incrementing a corresponding flow counter in the flow metering structure; a step for comparing the corresponding flow counter to a predetermined limit value; a step for, if the corresponding flow counter does not exceed the predetermined limit value, establishing the new VR flow, otherwise refusing to establish the new VR flow.
 13. The method of claim 9, wherein the processing engine comprises a Virtual Service Engine (VSE) configured to provide one or more services.
 14. The method of claim 13, wherein the VSE provides antivirus services.
 15. The method of claim 13, wherein the VSE provides firewall services.
 16. The method of claim 9, wherein the processing engine comprises an Advanced Security Engine (ASE) that provides data encryption services.
 17. A network device comprising: a means for maintaining a flow cache having a plurality of flow ID cache block entries each identifying one of a plurality of current virtual router (VR) flows through a VR-based network device and corresponding forwarding state information; a means for receiving an incoming packet at a processing engine of a plurality of processing engines of the VR-based network device; an ingress means, of a packet forwarding engine (PFE) associated with the processing engine, for determining whether the incoming packet is associated with a VR flow of the plurality of current VR flows by attempting to retrieve a flow ID cache block entry of the flow cache identified by an index based on one or more of (i) an Internet Protocol (IP) address, (ii) a Multiprotocol Label Switching (MPLS) label, and (iii) a destination logical queue (LQ) ID field, a LQ protocol ID field, one or more layer 3 (L3) header fields and one or more layer 4 (L4) header fields associated with the incoming packet; a means for, if it is determined that the incoming packet is associated with the VR flow, determining, based on the corresponding forwarding state information of the retrieved flow ID cache block entry, whether the incoming packet can be hardware forwarded or whether the incoming packet is to be software forwarded; a means for, if it is determined that the incoming packet can be hardware forwarded, (i) determining one or more packet transformations that are to be applied to the incoming packet by an egress unit of the PFE as a result of the incoming packet's association with the VR flow, (ii) the egress unit applying the one or more packet transformations to the incoming packet, and (iii) hardware forwarding the incoming packet without intervention by a processor of the VR-based network device via a network interface of the VR-based network device; otherwise, a means for, if it is determined that the incoming packet cannot be hardware forwarded, software forwarding the incoming packet via the processor; a means for, if it is determined that the incoming packet is not associated with any of the plurality of current VR flows, (i) identifying the existence of a new VR flow, (ii) allocating a new flow ID cache block entry within the flow cache for the new VR flow and (iii) forwarding the incoming packet to software on the processor for flow learning.
 18. The network device of claim 17, further comprising: a means for establishing a hardware accelerated flow within the flow cache, the hardware accelerated flow having associated therewith an identifier and an invalidation tag; a means for, if the packet is determined to be part of the hardware accelerated flow, retrieving a value from an invalid tag table by indexing into the invalid tag table with the identifier and comparing the invalidation tag to the retrieved value; and a means for invalidating the hardware accelerated flow when the retrieved value does not match the invalidation tag.
 19. The network device of claim 17, further comprising: a means for associating a rate metering structure with a VR flow of the plurality of current VR flows; a means for maintaining a rate statistic within the rate metering structure indicative of a number of packets or bytes associated with the VR flow that have been observed during a predefined time interval; and a means for enforcing a maximum packet or byte rate cap for the VR flow by dropping the packet if the packet would cause the rate statistic to exceed the maximum packet or byte rate cap for the predefined time interval.
 20. The network device of claim 19, further comprising: a means for associating a flow metering structure with a VR of a plurality of VRs of the VR-based network device; a means for incrementing a corresponding flow counter for each VR flow assigned to the VR in the flow metering structure; a means for comparing the corresponding flow counter to a predetermined limit value; a means for, if the corresponding flow counter does not exceed the predetermined limit value, establishing the new VR flow, otherwise refusing to establish the new VR flow.
 21. The network device of claim 17, wherein the processing engine comprises a Virtual Service Engine (VSE) configured to provide one or more services.
 22. The network device of claim 21, wherein the VSE provides antivirus services.
 23. The network device of claim 21, wherein the VSE provides firewall services.
 24. The network device of claim 17, wherein the processing engine comprises an Advanced Security Engine (ASE) that provides data encryption services.
 25. A virtual router (VR) based network device comprising: a flow cache having stored therein a plurality of flow ID cache block entries each identifying one of a plurality of current VR flows through the VR-based network device and corresponding forwarding state information; a processor operable to software forward packets that cannot be hardware forwarded and perform flow learning in relation to incoming packets not associated with any of the plurality of current VR flows a plurality of network interfaces through which incoming packets are received by and outgoing packets are transmitted by the VR-based network device; a plurality of processing engines coupled to the plurality of network interfaces, each of the plurality of processing engines having an associated packet forwarding engine (PFE) which has access to the flow cache; an egress unit associated with each of the PFEs operable to apply packet transformations to outgoing packets; an ingress unit associated with each of the PFEs operable to determine whether an incoming packet is associated with a VR flow of the plurality of current VR flows by attempting to retrieve a flow ID cache block entry of the flow cache identified by an index based on one or more of (i) an Internet Protocol (IP) address, (ii) a Multiprotocol Label Switching (MPLS) label, and (iii) a destination logical queue (LQ) ID field, a LQ protocol ID field, one or more layer 3 (L3) header fields and one or more layer 4 (L4) header fields associated with the incoming packet; and wherein if it is determined that the incoming packet is associated with the VR flow, then determining, based on the corresponding forwarding state information of the retrieved flow ID cache block entry, whether the incoming packet can be hardware forwarded or whether the incoming packet is to be software forwarded; if it is determined that the incoming packet can be hardware forwarded, then (i) determining one or more packet transformations that are to be applied to the incoming packet by the egress unit as a result of the incoming packet's association with the VR flow, (ii) the egress unit applying the one or more packet transformations to the incoming packet, and (iii) hardware forwarding the incoming packet without intervention by the processor via a network interface of the plurality of network interfaces; otherwise, if it is determined that the incoming packet cannot be hardware forwarded, then software forwarding the incoming packet via the processor; if it is determined that the incoming packet is not associated with any of the plurality of current VR flows, then (i) identifying the existence of a new VR flow, (ii) allocating a new flow ID cache block entry within the flow cache for the new VR flow and (iii) forwarding the incoming packet to software on the processor for flow learning.
 26. A program storage device readable by one or more processors of a virtual router (VR) based network device, tangibly embodying a program of instructions executable by the VR-based network device to perform method steps for routing traffic through the VR-based network device, said method steps comprising: maintaining a flow cache having a plurality of flow ID cache block entries each identifying one of a plurality of current virtual router (VR) flows through a VR-based network device and corresponding forwarding state information; receiving an incoming packet at a processing engine of a plurality of processing engines of the VR-based network device; an ingress unit of a packet forwarding engine (PFE) associated with the processing engine, determining whether the incoming packet is associated with a VR flow of the plurality of current VR flows by attempting to retrieve a flow ID cache block entry of the flow cache identified by an index based on one or more of (i) an Internet Protocol (IP) address, (ii) a Multiprotocol Label Switching (MPLS) label, and (iii) a destination logical queue (LQ) ID field, a LQ protocol ID field, one or more layer 3 (L3) header fields and one or more layer 4 (L4) header fields associated with the incoming packet; if it is determined that the incoming packet is associated with the VR flow, determining, then based on the corresponding forwarding state information of the retrieved flow ID cache block entry, whether the incoming packet can be hardware forwarded or whether the incoming packet is to be software forwarded; if it is determined that the incoming packet can be hardware forwarded, then (i) determining one or more packet transformations that are to be applied to the incoming packet by an egress unit of the PFE as a result of the incoming packet's association with the VR flow, (ii) the egress unit applying the one or more packet transformations to the incoming packet, and (iii) hardware forwarding the incoming packet without intervention by the one or more processors via a network interface of the VR-based network device; otherwise, if it is determined that the incoming packet cannot be hardware forwarded, then software forwarding the incoming packet via a processor of the one or more processors; if it is determined that the incoming packet is not associated with any of the plurality of current VR flows, (i) identifying the existence of a new VR flow, (ii) allocating a new flow ID cache block entry within the flow cache for the new VR flow and (iii) forwarding the incoming packet to the one or more processors for flow learning.
 27. The program storage device of claim 26, wherein the method further comprises: establishing a hardware accelerated flow within the flow cache, the hardware accelerated flow having associated therewith an identifier and an invalidation tag; if the packet is determined to be part of the hardware accelerated flow, then retrieving a value from an invalid tag table by indexing into the invalid tag table with the identifier and comparing the invalidation tag to the retrieved value; and invalidating the hardware accelerated flow when the retrieved value does not match the invalidation tag.
 28. The program storage device of claim 26, wherein the method further comprises: associating a rate metering structure with a VR flow of the plurality of current VR flows; maintaining a rate statistic within the rate metering structure indicative of a number of packets or bytes associated with the VR flow that have been observed during a predefined time interval; and enforcing a maximum packet or byte rate cap for the VR flow by dropping the packet if the packet would cause the rate statistic to exceed the maximum packet or byte rate cap for the predefined time interval.
 29. The program storage device of claim 28, wherein the method further comprises: associating a flow metering structure with a VR of a plurality of VRs of the VR-based network device; for each VR flow assigned to the VR, incrementing a corresponding flow counter in the flow metering structure; comparing the corresponding flow counter to a predetermined limit value; if the corresponding flow counter does not exceed the predetermined limit value, then establishing the new VR flow, otherwise refusing to establish the new VR flow. 